Environment Variables

U-Boot supports user configuration using environment variables which can be made persistent by saving to persistent storage, for example flash memory.

Environment variables are set using “env set” (alias “setenv”), printed using “env print” (alias “printenv”), and saved to persistent storage using “env save” (alias “saveenv”). Using “env set” without a value can be used to delete a variable from the environment. As long as you don’t save the environment, you are working with an in-memory copy. In case the Flash area containing the environment is erased by accident, a default environment is provided.

See env command for details.

Some configuration is controlled by Environment Variables, so that setting the variable can adjust the behaviour of U-Boot (e.g. autoboot delay, autoloading from tftp).

Text-based Environment

The default environment for a board is created using a .env environment file using a simple text format. The base filename for this is defined by CONFIG_ENV_SOURCE_FILE, or CONFIG_SYS_BOARD if that is empty.

The file must be in the board directory and have a .env extension, so assuming that there is a board vendor, the resulting filename is therefore:




This is a plain text file where you can type your environment variables in the form var=value. Blank lines and multi-line variables are supported. The conversion script looks for a line that starts in column 1 with a string and has an equals sign immediately afterwards. Spaces before the = are not permitted. It is a good idea to indent your scripts so that only the ‘var=’ appears at the start of a line.

To add additional text to a variable you can use var+=value. This text is merged into the variable during the make process and made available as a single value to U-Boot. Variables can contain + characters but in the unlikely event that you want to have a variable name ending in plus, put a backslash before the + so that the script knows you are not adding to an existing variable but assigning to a new one:


This file can include C-style comments. Blank lines and multi-line variables are supported, and you can use normal C preprocessor directives and CONFIG defines from your board config also.

For example, for snapper9260 you would create a text file called board/bluewater/snapper9260.env containing the environment text.


    /* U-Boot script for booting */

    if [ -z ${tftpserverip} ]; then
        echo "Use 'setenv tftpserverip a.b.c.d' to set IP address."

    usb start; setenv autoload n; bootp;
    tftpboot ${tftpserverip}:
    /* Print a message when boot fails */
    echo CONFIG_SYS_BOARD boot failed - please check your image
    echo Load address is CONFIG_SYS_LOAD_ADDR

If CONFIG_ENV_SOURCE_FILE is empty and the default filename is not present, then the old-style C environment is used instead. See below.

Old-style C environment

Traditionally, the default environment is created in include/env_default.h, and can be augmented by various CONFIG defines. See that file for details. In particular you can define CONFIG_EXTRA_ENV_SETTINGS in your board file to add environment variables.

Board maintainers are encouraged to migrate to the text-based environment as it is easier to maintain. The distro-board script still requires the old-style environment but work is underway to address this.

List of environment variables

Some device configuration options can be set using environment variables. In many cases the value in the default environment comes from a CONFIG option - see include/env_default.h) for this.

This is most-likely not complete:


If set to “yes” (actually any string starting with 1, y, Y, t, or T) an image loaded with one of the commands listed below will be automatically started by internally invoking the bootm command.

  • bootelf - Boot from an ELF image in memory

  • bootp - boot image via network using BOOTP/TFTP protocol

  • dhcp - boot image via network using DHCP/TFTP protocol

  • diskboot - boot from ide device

  • nboot - boot from NAND device

  • nfs - boot image via network using NFS protocol

  • rarpboot - boot image via network using RARP/TFTP protocol

  • scsiboot - boot from SCSI device

  • tftpboot - boot image via network using TFTP protocol

  • usbboot - boot from USB device

If the environment variable autostart is not set to a value starting with 1, y, Y, t, or T, an image passed to the “bootm” command will be copied to the load address (and eventually uncompressed), but NOT be started. This can be used to load and uncompress arbitrary data.


Used to set the baudrate of the UART - it defaults to CONFIG_BAUDRATE (which defaults to 115200).


Delay before automatically running bootcmd. During this time the user can choose to enter the shell (or the boot menu if CONFIG_AUTOBOOT_MENU_SHOW=y):

  • 0 to autoboot with no delay, but you can stop it by key input.

  • -1 to disable autoboot.

  • -2 to autoboot with no delay and not check for abort

The default value is defined by CONFIG_BOOTDELAY. The value of ‘bootdelay’ is overridden by the /config/bootdelay value in the device-tree if CONFIG_OF_CONTROL=y.


The command that is run if the user does not enter the shell during the boot delay.


Command line arguments passed when booting an operating system or binary image


Name of the image to load with TFTP


Memory range available for image processing in the bootm command can be restricted. This variable is given as a hexadecimal number and defines lowest address allowed for use by the bootm command. See also “bootm_size” environment variable. Address defined by “bootm_low” is also the base of the initial memory mapping for the Linux kernel – see the description of CONFIG_SYS_BOOTMAPSZ and bootm_mapsize.


Size of the initial memory mapping for the Linux kernel. This variable is given as a hexadecimal number and it defines the size of the memory region starting at base address bootm_low that is accessible by the Linux kernel during early boot. If unset, CONFIG_SYS_BOOTMAPSZ is used as the default value if it is defined, and bootm_size is used otherwise.


Memory range available for image processing in the bootm command can be restricted. This variable is given as a hexadecimal number and defines the size of the region allowed for use by the bootm command. See also “bootm_low” environment variable.

bootstopkeysha256, bootdelaykey, bootstopkey

See README.autoboot


Location of the software update file on a TFTP server, used by the automatic software update feature. Please refer to documentation in doc/README.update for more details.


if set to “no” (any string beginning with ‘n’), “bootp” and “dhcp” will just load perform a lookup of the configuration from the BOOTP server, but not try to load any image.


if set this restricts the maximum address that the flattened device tree will be copied into upon boot. For example, if you have a system with 1 GB memory at physical address 0x10000000, while Linux kernel only recognizes the first 704 MB as low memory, you may need to set fdt_high as 0x3C000000 to have the device tree blob be copied to the maximum address of the 704 MB low memory, so that Linux kernel can access it during the boot procedure.

If this is set to the special value 0xffffffff (32-bit machines) or 0xffffffffffffffff (64-bit machines) then the fdt will not be copied at all on boot. For this to work it must reside in writable memory, have sufficient padding on the end of it for u-boot to add the information it needs into it, and the memory must be accessible by the kernel. This usage is strongly discouraged however as it also stops U-Boot from ensuring the device tree starting address is properly aligned and a misaligned tree will cause OS failures.


if set this is the address of the control flattened device tree used by U-Boot when CONFIG_OF_CONTROL is defined.


restrict positioning of initrd images: If this variable is not set, initrd images will be copied to the highest possible address in RAM; this is usually what you want since it allows for maximum initrd size. If for some reason you want to make sure that the initrd image is loaded below the CONFIG_SYS_BOOTMAPSZ limit, you can set this environment variable to a value of “no” or “off” or “0”. Alternatively, you can set it to a maximum upper address to use (U-Boot will still check that it does not overwrite the U-Boot stack and data).

For instance, when you have a system with 16 MB RAM, and want to reserve 4 MB from use by Linux, you can do this by adding “mem=12M” to the value of the “bootargs” variable. However, now you must make sure that the initrd image is placed in the first 12 MB as well - this can be done with:

setenv initrd_high 00c00000

If you set initrd_high to 0xffffffff (32-bit machines) or 0xffffffffffffffff (64-bit machines), this is an indication to U-Boot that all addresses are legal for the Linux kernel, including addresses in flash memory. In this case U-Boot will NOT COPY the ramdisk at all. This may be useful to reduce the boot time on your system, but requires that this feature is supported by your Linux kernel. This usage however requires that the user ensure that there will be no overlap with other parts of the image such as the Linux kernel BSS. It should not be enabled by default and only done as part of optimizing a deployment.


IP address; needed for tftpboot command


Default load address for commands like “bootp”, “rarpboot”, “tftpboot”, “loadb” or “diskboot”. Note that the optimal default values here will vary between architectures. On 32bit ARM for example, some offset from start of memory is used as the Linux kernel zImage has a self decompressor and it’s best if we stay out of where that will be working.




TFTP server IP address; needed for tftpboot command








controls which network interface is used first.


controls which interface is currently active. For example you can do the following:

=> setenv ethact FEC
=> ping # traffic sent on FEC
=> setenv ethact SCC
=> ping # traffic sent on SCC

When set to “no” U-Boot does not go through all available network interfaces. It just stays at the currently selected interface. When unset or set to anything other than “no”, U-Boot does go through all available network interfaces.


When set to “no” each network operation will either succeed or fail without retrying. When set to “once” the network operation will fail when all the available network interfaces are tried once without success. Useful on scripts which control the retry operation themselves.


If set then Linux will be told to boot silently, by adding ‘console=’ to its command line. If “yes” it will be made silent. If “no” it will not be made silent. If unset, then it will be made silent if the U-Boot console is silent.


If this is set, the value is used for TFTP’s UDP source port.


If this is set, the value is used for TFTP’s UDP destination port instead of the default port 69.


Block size to use for TFTP transfers; if not set, we use the TFTP server’s default block size


Retransmission timeout for TFTP packets (in milli- seconds, minimum value is 1000 = 1 second). Defines when a packet is considered to be lost so it has to be retransmitted. The default is 5000 = 5 seconds. Lowering this value may make downloads succeed faster in networks with high packet loss rates or with unreliable TFTP servers.


maximum count of TFTP timeouts (no unit, minimum value = 0). Defines how many timeouts can happen during a single file transfer before that transfer is aborted. The default is 10, and 0 means ‘no timeouts allowed’. Increasing this value may help downloads succeed with high packet loss rates, or with unreliable TFTP servers or client hardware.


if this is set, the value is used for TFTP’s window size as described by RFC 7440. This means the count of blocks we can receive before sending ack to server.


When set to a value < 4095 the traffic over Ethernet is encapsulated/received over 802.1q VLAN tagged frames.

Note: This appears not to be used in U-Boot. See README.VLAN.


Period during which BOOTP/DHCP sends retries. Unsigned value, in milliseconds. If not set, the period will be either the default (28000), or a value based on CONFIG_NET_RETRY_COUNT, if defined. This value has precedence over the value based on CONFIG_NET_RETRY_COUNT.


Number of matches found by the last ‘ms’ command, in hex


Address of the last match found by the ‘ms’ command, in hex, or 0 if none


Index position of the last match found by the ‘ms’ command, in units of the size (.b, .w, .l) of the search


(x86 only) Base address of the bzImage ‘setup’ block


(x86 only) Address of the loaded bzImage, typically BZIMAGE_LOAD_ADDR which is 0x100000

Image locations

The following image location variables contain the location of images used in booting. The “Image” column gives the role of the image and is not an environment variable name. The other columns are environment variable names. “File Name” gives the name of the file on a TFTP server, “RAM Address” gives the location in RAM the image will be loaded to, and “Flash Location” gives the image’s address in NOR flash or offset in NAND flash.

Note - these variables don’t have to be defined for all boards, some boards currently use other variables for these purposes, and some boards use these variables for other purposes.

Also note that most of these variables are just a commonly used set of variable names, used in some other variable definitions, but are not hard-coded anywhere in U-Boot code.


File Name

RAM Address

Flash Location

Linux kernel




device tree blob








When setting the RAM addresses for kernel_addr_r, fdt_addr_r and ramdisk_addr_r there are several types of constraints to keep in mind. The one type of constraint is payload requirement. For example, a device tree MUST be loaded at an 8-byte aligned address as that is what the specification requires. In a similar manner, the operating system may define restrictions on where in memory space payloads can be. This is documented for example in Linux, with both the Booting ARM Linux and Booting AArch64 Linux documents. Finally, there are practical constraints. We do not know the size of a given payload a user will use but each payload must not overlap or it will corrupt the other payload. A similar problem can happen when a payload ends up being in the OS BSS area. For these reasons we need to ensure our default values here are both unlikely to lead to failure to boot and sufficiently explained so that they can be optimized for boot time or adjusted for smaller memory configurations.

On different architectures we will have different constraints. It is important that we follow whatever documented requirements are available to best ensure forward compatibility. What follows are examples to highlight how to provide reasonable default values in different cases.

Texas Instruments OMAP2PLUS (ARMv7) example

On these families of processors we are on a 32bit ARMv7 core. As booting some form of Linux is our most common payload we will also keep in mind the documented requirements for booting that Linux provides. These values are also known to be fine for booting a number of other operating systems (or their loaders). In this example we define the following variables and values:


The first thing to keep in mind is that DRAM starts at 0x80000000. We set a 32MiB buffer from the start of memory as our default load address and set kernel_addr_r to that. This is because the Linux zImage decompressor will typically then be able to avoid doing a relocation itself. It also MUST be within the first 128MiB of memory. The next value is we set fdt_addr_r to be at 128MiB offset from the start of memory. This location is suggested by the kernel documentation and is exceedingly unlikely to be overwritten by the kernel itself given other architectural constraints. We then allow for the device tree to be up to 512KiB in size before placing the ramdisk in memory. We then say that everything should be within the first 256MiB of memory so that U-Boot can relocate things as needed to ensure proper alignment. We pick 256MiB as our value here because we know there are very few platforms on in this family with less memory. It could be as high as 768MiB and still ensure that everything would be visible to the kernel, but again we go with what we assume is the safest assumption.

Automatically updated variables

The following environment variables may be used and automatically updated by the network boot commands (“bootp” and “rarpboot”), depending the information provided by your boot server:




see above


IP address of your Domain Name Server


IP address of your secondary Domain Name Server


IP address of the Gateway (Router) to use


Target hostname


See above


Subnet Mask


Pathname of the root filesystem on the NFS server


see above

Special environment variables

There are two special Environment Variables:


contains hardware identification information such as type string and/or serial number


Ethernet address. If CONFIG_REGEX=y, also eth*addr (where * is an integer).

These variables can be set only once (usually during manufacturing of the board). U-Boot refuses to delete or overwrite these variables once they have been set, unless CONFIG_ENV_OVERWRITE is enabled in the board configuration.



Contains the U-Boot version string as printed with the “version” command. This variable is readonly (see CONFIG_VERSION_VARIABLE).

Please note that changes to some configuration parameters may take only effect after the next boot (yes, that’s just like Windows).

External environment file

The CONFIG_USE_DEFAULT_ENV_FILE option provides a way to bypass the environment generation in U-Boot. If enabled, then CONFIG_DEFAULT_ENV_FILE provides the name of a file which is converted into the environment, completely bypassing the standard environment variables in env_default.h.

The format is the same as accepted by the mkenvimage tool, with lines containing key=value pairs. Blank lines and lines beginning with # are ignored.

Future work may unify this feature with the text-based environment, perhaps moving the contents of env_default.h to a text file.


See Environment implementation for internal development details.